1. Field of the Invention
The present invention relates to a gas barrier film having superior gas barrier performance and a method for producing the same as well as an organic electroluminescence device (henceforth referred to as “organic EL device”) utilizing the gas barrier film. More precisely, the present invention relates to a gas barrier film suitable as a substrate of various kinds of devices and a method for producing the same, as well as a flexible organic EL device utilizing the gas barrier film.
2. Description of the Background
Conventionally, gas barrier films prepared by forming a thin film of metal oxide such as aluminum oxide, magnesium oxide or silicon oxide on a surface of a plastic substrate or film have been widely used in packaging of articles which require shielding of various gases such as water vapor and oxygen, and packaging use for preventing deterioration of food, industrial materials, medical supplies and so forth. In addition to the packaging use, gas barrier films are recently coming to be used also in liquid crystal display devices, solar cells, EL devices and so forth.
In the course of development of image display devices such as liquid crystal display devices and EL devices in recent years, transparent base materials for forming these devices are required to satisfy highly sophisticated requirements in addition to suitability for production of lighter and larger devices, for example, they must have long term reliability and higher degree of freedom of the shape, they must enable display on a curved surface, and so forth. As a transparent base material that satisfies such sophisticated requirements, plastic base materials are beginning to be adopted as new base materials as an alternative to conventional glass substrates, which are heavy, readily broken and difficult to be formed with a larger size. Plastic base materials not only satisfy the aforementioned requirements, but also show more favorable productivity compared with glass substrates because a roll-to-roll system can be used for them, and therefore they are more advantageous also in view of cost reduction.
However, plastic base materials have a drawback that their gas barrier property is inferior to that of glass base materials. If a base material having poor gas barrier property is used, water vapor and air permeate the material to, for example, degrade liquid crystals in a liquid crystal cell, form display defects and thereby degrade display quality. In order to solve this problem, plastic films in which a metal oxide thin film is formed on a plastic film have been developed so far. Examples of such plastic films include those comprising a plastic film on which silicon oxide is vapor-deposited (Japanese Patent Publication (KOKOKU) No. 53-12953), those comprising a plastic film on which aluminum oxide is vapor-deposited (Japanese Patent Laid-open Publication (KOKAI) No. 58-217344) and so forth are known. These films have a water vapor barrier property of about 1 g/m2/day represented in terms of permeability.
The gas barrier performance required for plastic film substrates used for recently developed large-sized liquid crystal displays, high precision displays and so forth is about 0.1 g/m2/day in terms of water vapor barrier property. Furthermore, development of organic EL displays, high precision color liquid crystal displays and so forth has recently progressed, and therefore required are transparent base materials usable for these having further higher barrier performance, in particular, barrier performance of less than 0.1 g/m2/day in terms of water vapor barrier ability, while maintaining transparency. In order to meet such a demand, recently studied is film formation by the sputtering method or CVD method as a means that can be expected to provide higher barrier performance, in which a thin film is formed by using plasma generated by glow discharge under a low pressure condition. Moreover, organic luminescence devices in which a barrier film having an alternate laminate structure of organic layers and inorganic layers is produced by the vacuum deposition method are proposed (e.g., International Patent Application Laid-open Publication in Japanese (KOHYO) No. 2002-532850 (claim 1, FIG. 1)).
However, in these methods for forming a thin film, an organic substance blown as a vapor of a high temperature condenses on a film and form a thin film, and the film is temporarily heated and causes partial deformation. As a result, the subsequent lamination step becomes uneven, and thus the methods have a problem that they cannot provide sufficient barrier ability. Moreover, in these methods for forming a thin film, the number of the lamination step increases, and thus they have a problem of high cost. Furthermore, when a surface of a plastic substrate showing high water absorption is treated, the absorbed moisture is vaporized, thus it requires long time to obtain vacuum, and therefore there is also a problem that the processing cost becomes high.
Further, as for heat resistance of base material films, for disposing TFT in the production of active matrix type image devices in recent years, heat resistance of a still higher level is required. For example, a method of forming a polycrystalline silicon film at a temperature of 300° C. or lower by plasma decomposition of a gas containing SiH4 is disclosed in Japanese Patent Laid-open Publication (KOKAI) No. 7-81919 (claim 3, [0016] to [0020]). Moreover, a method of forming a semiconductor layer mixed with amorphous silicon and polycrystalline silicon on a polymer substrate at a temperature of 300° C. or lower by irradiation of an energy beam is disclosed in International Patent Application Laid-open Publication in Japanese No. 10-512104 [pages 14 to 22, FIG. 1, FIG. 7). Furthermore, a method of forming a polycrystalline silicon semiconductor layer on a plastic substrate provided with a thermal buffer layer at a temperature of 300° C. or lower by irradiation of a pulsed laser beam is disclosed in Japanese Patent Laid-open Publication No. 11-102867 (claims 1 to 10, [0036]).
As described above, various methods of forming a polycrystalline silicon film for TFT at a temperature of 300° C. or lower have been proposed, and therefore it is useful that the base materials have heat resistance for a temperature of 250° C. or higher. On the other hand, such methods for forming a semiconductor layer as described above use complicated configurations and apparatuses and require high cost, and therefore heat resistance for a temperature of 300° C. to 350° C. or higher is desired for plastics substrates.
U.S. Pat. No. 6,492,026 discloses a heat-resistant substrate having a barrier stack consisting of at least one inorganic layer and at least one polymer layer but it fails to show sufficient heat resistance.